A polymerizable liquid crystal compound has been recently utilized in an optical anisotropic substance for a polarizing plate, a reflective polarizer, a phase difference plate or the like because the compound exhibits optical anisotropy in a liquid crystal state and the anisotropy is immobilized by polymerization. Optical characteristics required for the optically anisotropic substance are different depending on a purpose, and therefore a compound having characteristics fit for the purpose is required. The compound used for the optically anisotropic substance has difficulty in controlling the anisotropy in the form of a single substance in many cases, and therefore may be occasionally utilized in the form of a composition in combination with various compounds.
The present inventors have developed a polymerizable liquid crystal composition utilizing an optically active compound having a binaphthol moiety and a polymerizable liquid crystal compound having a fluorene skeleton (Patent literature No. 1). The polymerizable liquid crystal composition containing the compounds can be applied in various uses by controlling a kind or an amount of addition of the optically active compound to change a helical pitch of an optically anisotropic substance having twist alignment. The optically anisotropic substance having twist alignment reflects light corresponding to a length of the helical pitch and a direction of rotation of a helix. Visible light is reflected when the length of the helical pitch is in the range of 380 nanometers to 780 nanometers. Near infrared light is reflected when the length of the helical pitch is longer than 780 nanometers, and ultraviolet light is reflected when the length of the helical pitch is shorter than 380 nanometers. Stacking or lamination of optically anisotropic substances having different directions of rotation allows total reflection. The optically anisotropic substance having such characteristics is referred to as a negative C plate.
In a liquid crystal display device in which a dark display (dark state) is obtained in a state in which liquid crystal molecules having positive birefringence are aligned perpendicularly to a substrate, no birefringence by alignment of the liquid crystal molecules is caused relative to a normal direction of the display device. Therefore, very high contrast is obtained in the normal direction in the above display devices. However, when the alignment is deviated from the normal direction of the display device, birefringence is caused, and transmittance of the black display (dark state) increases. More specifically, contrast decreases relative to a viewing angle in an oblique direction in the above liquid crystal display devices. The negative C plate can compensate the birefringence to be caused when the liquid crystal alignment is deviated from the normal direction in such a display device. As a result, the negative C plate serves as an optical compensation plate suitable for improving viewing angle characteristics in liquid crystal display devices including a vertically aligned (VA), twisted nematic (TN), optically compensated birefringence (OCB) or hybrid aligned nematic (HAN) liquid display device.
The optically anisotropic substance having the characteristics of the negative C plate described above is desired to cause no change in a phase difference or a reflection wavelength in various use environments. However, when the composition described in Patent literature No. 1 is used, optical characteristics have been deteriorated in several cases in an environment in which the device is exposed to a hot environment or brought into contact with a high polar solvent.
Patent literature No. 2 describes a combination of a photopolymerization initiator having oxime ester, and a polymerizable liquid crystal compound having fluorene structure. In the literature, an object is to increase birefringence (Δn) of an optically anisotropic substance in which polymerizable liquid crystal compounds are horizontally or vertically aligned by introducing a polymerizable liquid crystal compound containing a cinnamate bond. However, no attempt is described on improving durability of the optically anisotropic substance obtained therein. Moreover, Patent literature Nos. 3 to 4 on an optically active compound having a binaphthol moiety also provides no description on a combination with a photopolymerization initiator having an oxime ester group.
Patent literature No. 5 describes a combination of a photoinitiator having oxime ester, a non-polymerizable liquid crystal composition and a non-liquid crystalline polymerizable compound. However, the art relates to a material for forming a light controlling layer for a light scattering type liquid crystal device, and is essentially different from the invention. Patent literature No. 6 describes a combination of a photopolymerization initiator having a carbazole skeleton and an oxime ester moiety, an achiral polymerizable liquid crystal compound and an optically active compound having no binaphthol moiety. However, the art has had a problem of no capability of maintaining uniformity of twist alignment in the case of forming a negative C plate having length shorter than 380 nanometers in a length of a helical pitch of an optically anisotropic substance.